Bit Load Requirements for Geotechnical Bits

Unlike impregnated diamond and surface-set diamond products, Geotechnical bits set with either thermally stable polycrystalline (TSP) diamond elements or Pax (PCD) cutters require the use of high, sustained bit loads.

Geotechnical core bits offer rates of penetration that are considerably higher than impregnated diamond or surface-set diamond bits. In order to achieve these high rates of penetration, it is necessary to apply relatively high bit loads in order to maintain the sharpness of the individual TSP or Pax (PCD) cutters, particularly in medium formations. Insufficient bit loads will cause the cutter elements to become glazed and polished and as a result, the bit penetration will be reduced to virtually zero. Bit loads should not be so high as to exceed the manufacturer’s recommended maximum load rating for the core barrel and drill string in use.

For Geotechnical Core Bits set with Triangular-TSP or Cubic-TSP Elements:
The recommended bit load is approximately 100 to 125 lbs. force (450 to 550 N) per TSP Cutter mounted on the face of the bit.

For Geotechnical Core Bits set with Pax (PCD) Cutters:
The recommended bit load is approximately 500 to 600 lbs. force (2 200 to 2 700 N) per Pax (PCD) Cutter mounted on the face of the bit.

Bit Load Requirements for Tungsten-Carbide Set Bits

These types of bits are normally applied in extremely soft, abrasive and loose conditions and as such require very low bit loads coupled with very low rotational speeds. Due to the variable nature of the formations in which these types of bits are used, great care should be taken in applying bit loads. As always, the lowest possible bit load should be applied that will provide an acceptable rate of penetration.

For “Carbide-Chip” Type Tungsten-Carbide Core Bits

Bit load is a function of the bit face bearing area, that is, the actual contact area of the bit face with the formation being drilled. The maximum recommended bit load per bit face bearing area is 500 lbs / inch2 or (35 kg /cm2).

The formula used to calculate the maximum bit load (F) for any “carbide-chip” tungsten-carbide core bit is as follows:

Where:

“ F “ represents the calculated maximum bit load in “pounds force”.

“ D “ represents the outside set diameter of the bit crown in “inches”.

“ d “ represents the inside set diameter of the bit crown in “inches”.

“ n “ represents the number of full waterway canals across the bit face kerf.

“ w “ represents the typical width of the waterway canals in “inches”.

Based on the equation given above and on the maximum recommended bit loads per unit bit face bearing area, the recommendations for bit load (F) or “weight on bit” (WOB) for some common sizes of “carbide-chip” type tungsten-carbide core bits are given in the table below:

For "Saw-Tooth" Type Tungsten-Carbide Core Bits".

Bit load is a function of the total length of the tungsten-carbide cutters mounted on the bit face. Each of the tungsten-carbide cutters possesses a single cutting edge. The maximum bit load is determined by finding the sum of the linear length of cutting edge on all of the cutters on the bit face and multiplying that sum by a factor of load per unit length of cutting edge.

The formula used to calculate the maximum bit load (F) for any “saw-tooth” type tungsten-carbide core bit is as follows:

Where:

“ F “ represents the calculated maximum bit load in “pounds force”.

“ D “ represents the outside set diameter of the bit crown in “inches”.

“ d “ represents the inside set diameter of the bit crown in “inches”.

“ n “ represents the number of tungsten-carbide cutters mounted on the bit face.

Based on the equation given above, the recommendations for bit load (F) or “weight on bit” (WOB) for some common sizes of “saw-tooth” type tungsten-carbide core bits are given in the table below: